The present invention relates specifically to an artificial satellite, which is usable in the field of communications, such as satellite communications and mobile communications, a satellite orbit control method and a communication system using the satellite.
There is a requirement to transfer medical information including image data, for an emergency case carried on an ambulance from the ambulance to a paramedic center, and to direct medical treatment suitable for the emergency case to the ambulance from the doctors on duty at the paramedic center.
However, in the case of trying to transfer large-sized data, like image files, satisfactory results can not obtained by the conventional ground-base communication infrastructure. In addition, in case of communicating via geostationary satellites currently in service and/or mobile communication satellites to be deployed in the future, it is difficult to establish stable and continuous transfer lines extended from moving bodies because of shielding objects such as building structures and trees.
Though it is certainly possible to transfer large-sized data from movable objects, like an automobile, by using satellites moving in a specific orbit, no definite method for defining such an orbit has been established to date. Therefore, the orbit-related elements of such a specific orbit have not been definitely identified as yet.
Conventional technologies and their problems are described below in detail.
(A-1) Technologies and Problems in Ground-base Communication Infrastructures
In a case where large-sized data, like image files, are transferred from the movable bodies like automobile to a ground-base fixed station, communication methods via ground-base communication infrastructures or communication satellites can be considered. However, existing communication methods may not satisfy all the requirements for the system specification and performance.
Now, let""s take as an example an ambulance. For carrying an emergency case by ambulance, the average carrying time period is about 27 minutes. For serious cases, there occurs many instances in which the emergency case may die if adequate medical treatment is not applied in time, which is a strong motivation for the medical specialist to apply medical treatment to the emergency case in the ambulance or to suggest an adequate method for medical treatment to the emergency case to the emergency medical technician in the ambulance. However, about 15,000 or more medical doctors would be required for paramedic services in order to dispatch medical doctors with shift work to the about 5,000 ambulances in Japan. However, this is not realistic, and so it is considered to be more effective to communicate adequate methods for medical treatment from the paramedic center to the ambulance. However, in the conventional ground-base communication systems, communication lines with phone-level quality with which an instantaneous break may occur frequently are only available, and therefore, adequate methods for the medical treatment can not be directed satisfactorily from the paramedic center. If image information captured by endoscope, electrocardiogram, echo and camera could be directly transferred to the paramedic center, it is supposed that satisfactory diagnosis and directions for medical treatment of the emergency case could be given. However, ground-base communication infrastructures have such problems as limitation of transmission band, limitation of communication coverage areas, cross talk and interference due to reflection by man-made building structures, and so can not be applied to practical use for such a purpose.
Similarly, though many requests exist for large-scale data transmission from movable bodies, for example, live telecast of a marathon, the conventional ground-base communication infrastructure can not be used for this application.
(A-2) Technologies and Problems in a Geostationary Communication Satellite System
In the field of satellite communications using artificial satellites, communication systems using geostationary satellites and low-to-middle altitude orbits are well known. There are the following problems in conventional communication satellites.
As a geostationary satellite has about a 24-hour orbit cycle almost equal to the earth""s rotation cycle, the geostationary satellite can be viewed from the ground to be stationary at a point above the Equator. However, the elevation angle of such a geostationary satellite is low, for example, the elevation angle at Tokyo is at most 45 degrees even in case of good conditions. As the movable bodies in metropolitan areas move on the roads surrounded by artificial building structures and roadside trees, the lower range of the elevation angle is blocked by those obstacles, and satellite communications with geostationary satellites may be blocked. As the stationary satellites can be seen in an east-south to west-south direction, though communication lines can be established in a case where the movable body moves in a north-to-south direction and a broader visual field to the satellite can be obtained, communication lines may be blocked by building structures and roadside trees at almost any time in a day in a case where the movable body moves in an east-to-west direction, especially, in a west direction. Therefore, satellite communications using qeostationary satellites do not produce satisfactory results for service in not-plain areas, like a metropolitan area and a mountain area.
In the case of satellite communication systems using low-to-middle altitude orbits, such as Iridium and Odyssey currently under development for the purpose of cellular phone services using mobile communication satellites, the duration of time while the satellite in service stays within a high elevation angle range and comes in sight from the ground is generally short due to the limitation on the number of orbital planes for the satellite and the number of satellites in service. Especially, since a satellite flying on the low altitude orbit has about 90 to 100 minutes in its orbit cycle, the duration of time while the satellite stays within a high elevation angle range as viewed from the ground is as short as a few minutes. Therefore, when trying to use or apply this kind of satellite communication systems for the purpose of stable and definite communication for large-scale data, as used in the above example of an ambulance and a paramedic service system, without any influence by building structures, plants and natural topographic features over a certain extended time period, for example, more than 27 minutes, it is required to configure such a system using plural satellites which alternately may come in sight at a higher elevation angle. In this case, some thousand or more satellites are required, which causes difficulties in procuring a number of satellites, the operation thereof and launching cost reduction, and so this plan is not practical also from an economical point of view.
In the case where a higher elevation angle is required, as in the above example, conventional geostationary satellites for practical use and low-to-middle altitude satellites currently under development are not fully applicable.
For example, as found in research reports, such as xe2x80x9cFeasibility of Mobile Communication Mission Using NonGeostationary Satellite Orbitsxe2x80x9d, Technical Research Report, Japanese Electronics, Information and Communication Society, Vol. 89, No.57, satellite communication systems currently understudy are discussed. Especially, an oblong orbit having a larger eccentricity squared is proposed in some research reports including the above report.
According to Kepler""s Law, an object passing around the apogee point of the orbit slows down. By defining an orbit having its apogee point located on the upper air of the target service area, the duration time during which the satellite on this orbit stays at a high elevation angle can be taken to be long enough. Therefore, it is necessary to use an oblong orbit in order to establish communication lines for a extended period of time without a communication break due to building structures, roadside trees and natural geographical conditions.
As an example of oblong orbits, the Molnia orbit having about a 12-hour orbit cycle, a perigee altitude of some hundred km and an orbital inclination angle of about 63.4 degrees has been practically used as an orbit for communication satellites and military satellites in Russian territory since the 1960""s. Though this orbit is a stable orbit with its argument of perigee being fixed, and is certainly practical for service at the higher latitude locations over the Russian territory extended in a north and south direction, this orbit is not so practical for service at the lower latitude locations extended in a north and south direction, such as over Japan. Some orbits having about an 8-hour orbit cycle, about a 12-hour orbit cycle and about a 24-hour orbit cycle are proposed for the services provided in the Japanese territory. However, those proposed orbits are designed with localized optimization, and, as the orbits suitable for the north-to-south and east-to-west extension of the Japanese territory, there has not been any proposal for optimized orbits, methods for defining those orbits and definite operation technologies. This is because the design methodology for definition of a satellite orbit has been empirical in order to determine six orbit-related elements.
There are various methodologies for identifying and defining orbits, but the following six orbit-related elements are mainly used. Those are defined for an individual reference time.
Semi-Major Axis a: semi-major axis of the ellipse (noted by symbol 54 in FIG. 5),
Eccentricity Squared e: flatness of the ellipse orbital
Inclination Angle I: angle defined between the orbital plane and the equational plate
Right Ascension of North-Bound Node xcexa9: angle (shown by symbol 63 in FIG. 6) measured in the east direction from the vernal equinoctial point to the crossing point of the orbit from the northern hemisphere to the southern hemisphere with the equational plate (this crossing point shown by symbol 62 shown in FIG. 6)
(0 degreexe2x89xa6xcexa9xe2x89xa6360 degrees)
Argument of Perigee xcfx89: angle measured between the perigee and the right ascension of north-bound node 62 on the orbital plane (shown by symbol 63 in FIG. 6)
(0 degreexe2x89xa6xcfx89xe2x89xa6360 degrees)
True Anomaly xcex8: angle defined by the line connected between the perigee and the focal point of the ellipse and the line connected between the satellite and the focal point of the ellipse (shown by symbol 58 in FIG. 5)
(0 degreesxe2x89xa6xcex8xe2x89xa6360 degrees).
The geometrical relationship for those elements will be described with reference to FIGS. 5 and 6. The satellite 51 moves on the elliptical orbit having a focal point 50. The distance between the perigee 53 of the ellipse and the focal point 50 of the ellipse is represented by perigee radius Rp and with symbol 57 in FIG. 5. The distance between the apogee 52 of the ellipse and the focal point 50 of the ellipse is represented by apogee radius Ra and with symbol 56 in FIG. 5. Perigee radius, apogee radius, semi-major axis a represented by symbol 54 in FIG. 5, semi-minor axis b represented by symbol 55 in FIG. 5 and the eccentricity squared e have the following relations.
Rp=a(1xe2x88x92e)
xe2x80x83Ra=a(1+e)
B=a(1xe2x88x92e2)1/2
e=(Raxe2x88x92Rp)/(Ra+Rp)
In FIG. 6, what is shown is an example in which the earth 60 is positioned at the focal point of the elliptical orbit. The elliptical orbit crosses at the north-bound node 62 on the equational plate from the southern hemisphere to the northern hemisphere, while the perigee is positioned at the point 65 and the apogee is positioned at the point 66. The angle 64 between the equational plate 61 and the orbital plane defines the orbital inclination angle i. The right ascension of the north-bound node is defined by the angle 68 measured in the eastern direction from the vernal equinoctial point, and the argument of the perigee is defined by the angle 63 between the north-bound node 62 and the perigee 65.
Even if the semi-major axis can be specified definitely by the orbit cycle, other major parameters may be determined to be arbitrary values, such as the eccentricity squared is an arbitrary real number 0.0 or over and less than 1.0, the orbital inclination angle is an arbitrary real number 0.0 degree or over and 180 degrees or smaller, and the argument of perigee is an arbitrary real number 0.0 degree or over and 360 degrees or smaller. Thus, there may occur a situation in which a designer is forced to determine values for those parameters intuitively and/or empirically from his or her experiences.
If a satellite which can come in sight in the zenith direction for an extended period of time on the upper air of the target service area can be realized, xe2x80x9clarge-scale data transfer from mobile bodies for an extended period of timexe2x80x9d can be established by satellite communications. Thus, what has been sought are feasible methodologies for defining orbit-related elements and their definite values which can be adaptive to Japanese territory characteristics and are cost-effective, that is, configured with less number of satellites forming the overall system.
As described above, in order to transfer large-scale data including image files from movable bodies, like an automobile, for an extended period of time, it is required to make the satellite remain on the orbit in the zenith direction as long as possible and to communicate with the satellite.
It has been generally recognized that it is preferable to establish an orbit shaped in an oblong ellipse having its apogee on the upper air of the target service area, in order to satisfy the above described requirement. However, adequate methodologies and algorithms for defining orbit-related elements have not been proposed. In addition, there is no definite proposal for specified values for those parameters to be optimized for the services over the whole Japanese land.
An object of the present invention is to provide a specific methodology for setting orbit-related parameters with respect to the above described problems, and to set the orbit-related parameters in terms of a limited range for the parameters obtained by this methodology.
Another object of the present invention is to provide various systems using artificial satellites arranged so as to be able to remain insight in the zenith direction for a long stretch of time in order to solve the above described problems.
And furthermore, another object of the present invention is to provide an orbit control means for performing the control of artificial satellite orbits based on the orbit-related parameters defined in the manner described above.
In order to achieve the above objects, in accordance with the present invention, in an artificial satellite traveling along an elliptical orbit, the elliptical orbit is defined by six orbit-related parameters obtained with input conditions, including the geographical condition of the service area to be covered by the artificial satellite, the tolerance of the ascending vertical angle within which the artificial satellite can be viewed from the service area, and the reference time defining the orbit elements.
The artificial satellite traveling on an elliptical orbit according to the present invention travels on such an oblong orbit that the artificial satellite may come in sight at an angle larger than the maximum elevation angle with which the geostationary artificial satellite is viewed from the service area corresponding to the artificial satellite.
As for the determination of orbit elements, six orbit related parameters are determined by steps including the step of setting the semi-major axis, the step of setting the perigee arguments, the step of setting the semi-vertical angle, the step of setting the desired service time, the step of setting the polygon including the service area, the step of setting the number of artificial satellites, the right ascension of the north-bound node of the individual artificial satellite and the true anomaly of the individual artificial satellite, the step of setting the initial value for the orbital inclination angle, the step of calculating the duration time for the artificial satellite coming into sight from the individual apex of the polygon, the step of setting the combination of the orbital inclination angle and the eccentricity squared and the step of resetting the right ascension of the north-bound node and true anomaly of the individual artificial satellite.
In order to achieve the above described object of the present invention, in the group of artificial satellites including plural artificial satellites traveling on elliptical orbits, six orbit-related parameters of the elliptical orbits of the individual artificial satellites are obtained with input conditions including the geographical condition of the service area to be covered by the artificial satellites, the tolerance of the ascending vertical angle within which any one of the group of artificial satellites can be viewed from the service area, and the reference time defining the orbit elements; and, what are used are a group of satellites such that one or more artificial satellites are arranged on the individual orbital planes within the predetermined range of the ascending vertical angle viewed in the zenith direction from the service area by combining plural elliptical orbits so that at least one or more artificial satellites can always come in sight.
In order to achieve the above described object of the present invention, artificial satellites with orbits provided by the present invention are used in various systems using artificial satellites, such as an orbit control system for controlling the orbit of satellites, a satellite communication system for conducting satellite communications with artificial satellites, and an earth observing system using artificial satellites carrying earth observing devices.
In case the satellite communication terminal in the satellite communication system is used within the service area covered by the artificial satellites of the present invention, the satellite communication terminal may have send/receive means for sending and receiving signals to and from the target artificial satellite coming in sight in the range of the ascending vertical angle in the predetermined zenith direction, and may be loaded on the movable body moving mainly within the service area. In addition, the satellite communication terminal may include GPS means for receiving radio waves from GPS satellites forming a global positioning system and at least for measuring the position of the satellite communication terminal itself, and may have measuring means for measuring the quantity consumed by every house to be charged for electricity, gas or public water supplied by public utility services.
In order to achieve the above described object of the present invention, in the group of artificial satellites including plural artificial satellites traveling in elliptical orbits, six orbit-related parameters of the elliptical orbits of the individual artificial satellites are obtained so as to satisfy the input conditions including the geographical condition of the service area to be covered by the artificial satellites, the tolerance of the ascending vertical angle within which any one of the group of artificial satellites can be viewed from the service area, and the reference time defining the orbit elements. In case plural artificial satellites are employed, one or more artificial satellites may be arranged on the individual orbital planes within the predetermined range of ascending vertical angle viewed in the zenith direction from the service area by combining plural elliptical orbits so that at least one or more artificial satellites can always come in sight.
The above described object is established by a orbit element determination apparatus comprising means for setting a polygon including a semi-major axis, a perigee argument, a semi-vertical angle, a service time and a service area; means for setting the number of satellite and right ascension of the north-bound node and true anomaly of the individual satellite; means for setting the initial value for the orbital inclination angle; means for calculating the duration time for the satellite coming into sight from each apex of the polygon; means for setting a combination of the orbital inclination angle and the eccentricity squared and means for resetting the right ascension of the north-bound node and the true anomaly of the individual satellite.
In order to achieve the above described object, in the satellite communication system for conducting satellite communications with artificial satellites, the present invention at least includes an artificial satellite, a satellite communication terminal for conducting satellite communications with artificial satellites and a base station for conducting communications to and from the satellite communication terminal with the artificial satellites, in which the artificial satellite is a satellite that travels on such an oblong orbit that the artificial satellite may come in sight at an angle larger than the maximum elevation angle with which the geostationary artificial satellite is viewed from the service area corresponding to the artificial satellite, and the satellite communication terminal may be loaded on a movable body and have send/receive means for sending and receiving signals to and from the target artificial satellite coming in sight in the range of the ascending vertical angle in the predetermined zenith direction when used within the service area covered by the artificial satellite.
In order to achieve the above described object, in the satellite communication system for conducting satellite communications with artificial satellites, the present invention at least has an artificial satellite and plural satellite communication terminals for conducting satellite communications with artificial satellites, in which the artificial satellite is a satellite that travels on such an oblong orbit that the artificial satellite may come in sight at an angle larger than the maximum-elevation angle with which the geostationary artificial satellite is viewed from the service area corresponding to the artificial satellite, and plural satellite communication terminals have send/receive means for sending and receiving signals to and from another satellite communication terminal, and at least one of the plural satellite communication terminals is located within the main service area, the other of the plural satellite communication terminals are located in the area outside the main service area and from which satellite communications with the artificial satellite are possible, and any one of the relay operations may be selected in response to the ascending vertical angle of the artificial satellite viewed from the main service area covered by the artificial satellite, in which relay operations include a relay operation between satellite communication terminals located within the main service area, a relay operation between the satellite communication terminal located within the main service area and the satellite communication terminal located in the other areas, and a relay operation between satellite communication terminals located in the area other than the main service area.